What is the big attraction of rhododendrons? The dramatic flowers that cover the entire shrub in springtime. Rhododendron flowers come in a range of colors to fit any garden design scheme. Gardeners choose rhododendrons primarily for the flowers. Making sure that the rhododendron blooms heavily, year after year, is a top priority. Three factors influence blooming, and gardeners control them all: location of the rhododendron in the garden, proper fertilizer and complete deadheading of spent blooms.
Find an area of your garden that provides some overhead protection for the rhododendron. Once the rhododendrons bloom, they are sensitive to frost. Overhead protection prevents damage from a sudden spring frost.
Plant the rhododendron where it will receive bright, filtered light. Morning sun and afternoon shade promote blooming.
Avoid locations that receive early morning sun in the winter. This heats the leaves while the roots are still cold in frozen ground, leading to poor water circulation, which can kill flower buds.
Fertilize the rhododendron with phosphorous (superphosphate or triple superphosphate) when planting or transplanting. Phosphorous cannot be applied from the surface. It is an important support for bud formation.
Avoid high-nitrogen fertilizers, which encourage vegetative growth rather than flowers. Cottonseed meal is an organic, and therefore slow-release, source of nitrogen. If the rhododendron leaves are green but blooms are few, try limiting even this form of fertilizer.
Apply potassium to promote hardiness in flower buds. Compound fertilizer numbers show the respective levels of nitrogen, phosphorous and potassium. Choose a fertilizer with a higher third number.
Deadheading after blooming
Put on garden gloves to protect your hands from the sticky honeydew produced by rhododendron blooms. Look at the faded blooms and note that the blooms grow in clusters or “trusses."
Find the stem of the truss and establish where it meets the new bud forming beneath it.
Pinch the bottom of the truss stem and snap it off or “deadhead” it. Be careful not to disturb the growing bud underneath.
Toss the spent truss in a yard waste container, to be added to your compost pile.
Continue to deadhead the entire rhododendron for maximum bloom the following season.